Automatic volume control means for facsimile systems



Oct. 14, 1941. I WEDJG JR 2,258,871

AUTOMATIC VOLUME CONTROL MEAITS FOR FACSIMILE SYSTEMS Filed Oct. 26,1958 IN VENTOR.

Ffmwc Z. WED/G; BY

ATTORNEYfi Patented Oct. 14, 1941 AUTOMATIC VOLUME CONTROL MEAN FORFACSIMILE SYSTEMS Frank L. Wedig, .n-L, Cincinnati, Ohio, assignor a,

The Crosley Corporation, Cincinnati, Ohio, a

corporation of Ohio Application October 26, 1938', Serial No. 237,107

10 Claims.

My invention has for its fundamental object the provision of a novelautomatic volume control for facsimile systems.

My invention relates to facsimile systems de signed to reproducepictures or other intelligence transmitted by radio. In an exemplary arrangement the copy is scanned by a photo-cell at the transmittingstation, and the picture signal thus produced is caused to modulate asubcarrier of, say, a frequency of 2000 cycles derived from a localoscillator. The modulated sub-carrier is then used to modulate aradiocarrier which is transmitted. At the receiving end, the carrier ispicked up, amplified and detected in any of the usual ways. Theresultant modulated sub-carrier is then rectified, preferably byhalf-wave rectification and is impressed upon a printing device.

My present invention is not limited to any particular printing means, ofwhich there are many, including means for printing upon photosensitizedpaper by means of "a variable light source. An exemplary printing meanscomprises a scanning arm which is caused bysuitable means to swing backand forth across a record strip. The armbears a stylus which contactsthe record strip. The record strip-rests upona platen. The record stripis of paper, rendered conductive as by being impregnated with carbon,and coated with a substance such as mercuric oxide which willchange-color upon the development of sufficient potential-differencebetween the stylus and the platen.

'The details of such facsimile system 'do not constitute limitationsupon the present invention, nor are they necessary toan'- understandingof it. however, set forth in the following-co-pending applicationsSilver, Ser. No. 237,105, filed Oct. 26, 1938, and

entitled Transmission and detection means and method for facsimilesystems. Silver and Wedig, Ser. No. 237,106,- filed Oct. 26, 1938, andentitled Means for improving the operation of facsimileprinters. a

Wedig, Ser. No. 237,108,'filed Oct. 26,1938, and entitled Humelimination in fac'simile'receivers. It will be understood that volumecontrol, 1.- e;

control of the general signal amplitude, is ex-- of the radio signalwill cause the blackness-of- Such systems or portions thereof are,-

" at the trol the volume. Thus it becomes vital to provide means forsetting the system so that in spite of fading, the signal will'always beof sufficient strength to print, and for automatically controlling themaximum signal level so that the record is not blackened all over orburned.

Since most facsimile means will be used with radio sets which are alsoused for broadcast reception and loud speaker reproduction, and sincefacsimile signals will usually be sent out after the broadcastingschedules for the day are finished so that it is desirable to providemeans for properly setting the facsimile system to the Proper signallevel before that level can be visually judged by inspection of a recordbeing reproduction, and second that the facsimile receiving system willusually be turned on and off time of facsimile broadcast periods by timeclock'mechanism.

Since most facsimile receivers and printers will be used with ordinaryradio sets not pro-'- vided with an adequate facsimile volume control,an object of my invention is the provision of a facsimile installationembodying a converter having the features of my invention, which may'easilyand quickly be coupled to an existing radio set withoutinterfering with its regular operation. 1 To return to a' considerationof the construction of facsimile systems as such, it will be understoodthat at the transmitting end, before the print to vary, giving animperfect 'reproduc- 1 tion. 0n large fades the printingmaystop'entirely. Moreover, the printing will usually be done when there isno operatorpresent to conthe start of each scanning stroke asynchronizing signal derived from a local oscillator of, say, afrequency of 500 cycles per second, is sent out. At the receiving end;the scanner aim, at the conclusion of each return stroke, is heldorrestrained until the synchronizing signal is received. Thussynchronism' is maintained be-' tween the scanning devices at thesending and receiving stations. a regularly and frequently recurringsignal --of constant and relatively high intensity. It is an object ofmy invention to provide an automatic volume control arrangement whichemploys the synchronizing signal as the means for generating the A. V.C; voltage for reasons just indicated, and which operates to vary thepicture signal level in accordance with variations in the Thesynchronizing signal is synchronizing signal level, as received, so asto maintain the constancy of both signal levels.

These and other objects of my invention which Will be set forthhereinafter or will be apparent to one skilled in the art upon readingthese specifications, I accomplish by that certain construction andarrangement of parts of which I shall now describe an exemplaryembodiment. Reference is made to the drawing which is a circuit diagramembodying the features of my invention.

In the drawing, the dotted line box I indicates an ordinary radioreceiver, of which I have shown a portion of the output circuits. 2represents an output tube feeding the primary of an output transformer3. The secondary of the transformer feeds the voice coil of a loudspeakor indicated generally at 4; and ordinarily the speaker circuit iscompleted by a connection indicated in dotted lines at 5. In installingthe facsimile system, this connection is broken and the transformersecondary and voice coil leads, formerly connected by the connection arebrought out toterminals 6 and 8. Another terminal 1 is connected intothe circuit intermediate the transformer secondary and voice coil asshown.

The other portions of the circuits shown in the drawing may beconsidered as part of an installation for facsimile to be added to theusual radio set, and may be located in a separate cabinet which alsohouses the facsimile device; or the facsimile device and the circuitsillustrated may be housed in different cabinets. In either event theseparate cabinet or cabinets may be placed on or near the radio set.Needless to say, in a radio set especially designed for facsimilereception all of the elements may be built into a single cabinet. A

The terminal 6 is connected bya lead 9 to one of the movable elements Itof a triple throw double pole switch indicated generally at H. Theterminal 8 is connected by a lead l2 to the terminal l3 of this switch.With the switch in the position shown, the voice coil is connected tothe secondary of the transformer 3 justas though the connection 5 hadnot been broken; and the circuit arrangements hereinafterto be describedare open. The position of the switch shown is therefore the position forregular broadcast reception.

The terminal I is connected by a lead I4 to one end of the primary [5 ofa transformer indicated generally at IS. The other end of the primary isconnected by a lead I! to contacts l8 and IQ of the switch aforesaid, sothat in each of the other positions of the switch, excepting the firstmentioned one, the loud speaker 4 will be cut out of the output circuitof the radio set I, and the'primary l5 cut into it.

The transformer l6 has a secondary 20, one end of which is connected toan electrode 21 of a tube 22. This electrode, together with thecathode23 of the tube, constitutes a diode combination. The cathode 23is grounded through the usual combination of resistance 24 and capacity25 as at 25. The other end of the secondary is connected 'to groundthrough a parallel combination of resistance 21 and capacity 28, aboutwhich'more will be saidghereinafter. The electrode2l is also connectedthrougha capacity 29, and theresistance element of a.

potentiometer, directly to the secondary 23 by a lead 3 I. The lead 3|is connected througha resistor 32 toa lead' 33 which is grounded was at34 through a capacity 35. The movable contact 36 is connected through acapacity 31 to the grid 38 of an exponential grid audio amplifier tube39. The cathode 40 of this tube is grounded as at 43 through the usualparallel combination of resistor 4| and capacity 42. The grid 38 is alsoconnected to the lead 33 through a resistor 44. The plate 45 of the tube39 is connected to the primary 46 of a transformer 41. The secondary 48of this transformer feeds the facsimile printer, or preferably a powerstage for the facsimile printer. It will be understood that thefacsimile printer has rectifying means in circuit with it.

The diode combination 2l--23 rectifies the audio signal and generates aD. C. bias across 21 and 28. This bias is put on the grid of the tube 39through resistances 32 and 44. The unrectified audio signal, flowingthrough the circuit formed by elements 20, 29, 30, and 3| is impressedon the grid-oftube 39 through the contact 36 of the potentiometer andthe capacity 31. The D. C. bias generated by the diode combination 2| to23 is thus used to' control the gain of the tube 39 so that the gain isinversely proportional to the signal.

i I have-already pointed out that the synchronizing signal astransmitted is a periodic signal of relatively high and constantintensity occurring at regular and frequent intervals. In order toemploy this signal to generate the D. C. bias for automatic volumecontrol purposes, I make the rectifying condenser 28 and the diode loadresistor21 large enough to have a certain time constant. This timeconstant is of suflicient value that the D. C. voltage generated by thediode will not change appreciably from one synchronizing pulse to thenext. In this way, a bias is developed for the amplifier tube 39 whichis Wholly determined by the synchronizing pulses.

It is advantageous to provide for delay on the automatic volume control.By this I mean that I provide means whereby the signal must reach apredetermined value before its intensity is cut down by the automaticvolume control means. The utility of this will at once be apparent. Thetube-22-also comprises a triode combination including the cathode 23, agrid 49, and a plate 50, which is connected to the positive side of theB source as indicated. The grid 49 is connected to the lead 33 by a-lead5!. The plate 50 draws current when no bias is applied to the grid. Thisplate current, passing through the resistor 24, biases thecathode 23with respect to the electrocle 2| of the diode combination. The signaloperates to bias the grid through the connections hereinabove,mentioned. When the signal becomes great enough to overcome the biasproduced by the plate circuit, automatic volume control voltageisdeveloped by the diode combination. This bias, applied to the grid 49,biases ofi the triode section of the tube 22, and thus removes thedelaybias generated as aforesaid.

In a facsimile-system there is an optimum level at which the receivershould work. This level is highenough to produce a good signal-to-humratio, yet mustnot beso high as to overload the radio output tube 2 andproduce distortion, nor so high-as to produce sufficient bias from thediode'to cutoffthe amplifier tube. I have already explained thenecessity, in'aninst'allation ofthe type I have been describing, forsome means for indicating to the operator, when the proper .adljustmenthas been attained. To this end, Iprovide onthe transformer l6, atertiary winding 52.

One' end of this winding maybe connected to the other. movable .member,53. of; the switch ll through a suitable indicating device. Theotherend of; the winding 52 is brought out to the middle contact 55 of theswitch. The first and third contacts 54 and 56 of the switch are blank.Any suitable indicating means may of course be used, including metersandthe like; but I have found it convenient to employ a small pilotlight bulb 51, and to. proportion the windings of the'transformer l6 sothat the bulb just lights at the proper audio level. This, as will beclear, enables the operator to adjust the audio level of the radio setprior to the commencement of the facsimile broadcast, as will usually bedesired. I V The automatic volume control which forms one aspect of myinvention will be in addition to any automatic volume control alreadypossessedby the radio set.

Modifications may be made in my invention without departing fromthespirit of it. 'Having thus described my invention, what I claim' as newand desire to secure by Letters Patent is: I u I.

1. In a facsimile system employing recurring synchronizing impulses,receiving means includ ing amplifying means, means for generating from areceived signal a direct current automatic volume control biasingvoltage, means for applying said voltage to said amplifying means, andtime delay-means in connection with'said generating means to prevent thebiasing'voltage so generated from diminishing substantially betweensaidsynchronizing impulses, and means in connection with said generatingmeans for preventing the generation of said biasing voltage until thesignal strength reaches a predetermined level, said means for generatingthe said biasing voltage being a diode rectifier, and said means forpreventing the generation of said voltage comprising thermionic meansfor biasing said diod with a voltage which must be overcome by thesignal voltage. a 4 v v 2. In a facsimile system employing recurringsynchronizing impulses,-receiving means including amplifying means,means for generating from areceived signal a direct current automaticvolume control biasing voltage, means for applying said voltage to saidamplifying means, and time delay means in connectionwith said generatingmeans to prevent the biasing voltage so generated from diminishingsubstantially'between said synchronizing impulses, and means inconnection with said generating means for preventing the generation ofsaid biasing voltage until the signal strength reaches a predeterminedlevel, said means for generating the said biasing voltage being a dioderectifier, and said means for preventing the generation of said voltagecomprising thermionic means for biasing said diode with .a voltage whichmust be overcome by the signal voltage, and a connection between saiddiode and said thermionic means such as to bias ofi said thermionicmeanswhen said diode generates said biasing voltage;

3. In a facsimile system employing recurring synchronizing impulses, areceiver, printing means, and automatic volume control means inconnection with said receiver for controlling the signal level appliedto said printing means in accordance with the intensity of receivedsynchronizing impulses only, said means comprising a circuit having adirect current path including a rectifier, and an alternating currentpath, n amplifier tube, and means for impressing. on tliegridfof saidamplifier tube voltages fromboth of said paths,the voltage from saidalternating current path comprising a signal voltage to beamplified, andthe voltage from said direct current path comprising a, biasing voltageto cause said: tube .to be responsive in inverse proportion to, thesignal strength, and means in said direct current path .to preventsubstantial diminution, of the generated biasing voltagethereinr'during'the interval between synchronizingimpulsesa I s f 24. Ina facsimile system employing'recurring synchronizing, impulses, .areceiver, printing means, and automatic. volume control means inconnection with said receiverfor controlling the signal level applied tosaid printingmeansin accordance with the intensity of received ynchronin mpulses only, said meanscomprising a circuit having a direct currentpath including a rectifier, and an alternating current path, anamplifier. tube, and meansfor impressing on the gridof said amplifiertubevoltages from both of said paths, the voltage from said alternatingcurrent path comprising a signal voltage to be amplified-, and,thevoltage from said direct current path comprisinga biasing voltage tocause said tube, to beresponsive, in inverse proportion to the signalstrength, and meansrin said direct current path to prevent substantialdiminution of the generated biasing, voltage therein during the intervalbetween synchronizing, impulses, said last mentioned means comprising aparallel 'ree sistanceandcapacity.combination in said direct currentpath,;having.sufiicientvalue to give the desired time delay; V t i 5. Ina facsimile system employing recurring synchronizing impulses, areceiver, printing means, andautomatic volume control means inconnection with said receiver for, controlling the signal level appliedto said printing means in accordance with the intensity of received synechronizing impulses only, aid means comprising a circuit having adirectcurrent path including a rectifier, and. an alternating current path, anamplifier tube, and means for-impressing onthe grid of; said,amplifiertube voltages from both of said paths; the voltage from saidalternating current path comprising a signal voltageto be amplified, andthe voltage from said direct current path comprising a biasing voltageto cause said tube to be responsive in inverse proportion to the signalsrength, means in said direct cure rent path to prevent;substantialdiminution of the generated biasing voltage therein during the intervalbetween synchronizing impulses. and means for preventingthe fiow ofdirect current in said direct current path until the attainment of asignal level of predetermined value. 6-.-In a facsimile system employingrecurring synchronizing V impulses, a receiver, printing means, andautomatic volume control means in connection with said receiver forcontrolling the signal level applied to said printing means inaccordance with the intensity of received synchronizing impulses only,said means comprising a circuit 'having'a direct current path includinga rectifier, and an alternating current path, an amplifier tube, andmeans for impressing on the grid of said amplifier tube voltages fromboth of said paths, the voltage from said alternating current pathcomprising a signal voltage to be amplified, and the voltage from saiddirect current path comprising a biasing voltage to cause said tube tobe responsive in inverse proportion to the signal strength, means insaid direct current path to prevent substantial diminutionof thegenerated biasing voltage therein during the interval betweensynehronizing impul'ses, and means for preventing the flow-ofdirectcurrent in said direct current path until the attainment of a signallevel of predetermined value, said last mentioned means comprisingthermionic means for impressing on said rectifier a bias to render-itinoperative in the absence of signal. s

7. In a facsimile. system employing recurring synchronizing impulses, areceiver, printing means, and automatic volume control means inconnection with said receiver .for controlling the signal level appliedto said printing means in accordance with the intensity of receivedsynchronizing impulses only, said means comprising a circuit having adirectcurrent path including a rectifier, and an alternating currentpath, an amplifier tube, and means for impressing on the grid of saidamplifier tube voltages from both of said paths, the voltage from saidalternating current path comprising a signal voltage to be amplified,and the voltage from said direct current path comprising a biasingvoltage to cause said tube to be responsive in inverse proportion to thesignal strength, means in said direct current path to preventsubstantialdiminution of the generated biasing voltage therein during the intervalbetween synchronizing impulses, and means for preventing the flow ofdirect current in said direct current path until the attainment of asignal level of predetermined value, said last mentioned meanscomprising thermionic means for impressing on said rectifier a bias torender it inoperative in the absence of signal, and a connection betweensaid thermionic means and said rectifier such as to cause direct currentin said direct current path to bias said thermionic means to cut off.

8. In a facsimilelinstallation for connection to a radio set, an inputtransformer having a primary and secondary, means for selectivelyconnecting said primary to said radio set and facsimile means inconnection with said secondary, a third winding on said transformer, andmeans in connection with said third winding for indicating a correctsignal level for facsimile reception, and switch means operating inconnection with said first mentioned switch means for placing saidindicating device in the circuit of said third winding when anadjustment is desired, there being amplifying means between saidfacsimile means and said secondary, and means for deriving an automaticvolume control voltage and for applying it to said amplifying means,located between said secondary and said amplifying means, said means forderiving an automatic volume control voltage comprising a dioderectifier in circuit with said secondary, means for applying directcurrent generated by said rectifier to said amplifying means, and meansin the circuit of said rectifier to cause said rectifier to beresponsive substantially only to recurring received synchronizingimpulses.

9.'In a facsimile installation for connection to a radio set, an inputtransformer having a primary and secondary, means for selectivelyconnecting said primary to said radio set and facsimllemeansinconnection with said secondary, a third'winding .on said transformer,and means inconnection with said third winding for indieating a correctsignal level for facssimile reception, and switch means operating inconnection with said first mentioned swicth means for placing saidindicating device in the circuit .of said third winding when anadjustment is desired, there being amplifying means between saidfacsimile means and said secondary, and means for deriving an automaticvolume control voltage and for applying it to said amplifying means,located between said secondary and said amplifying means, said means forderiving an automatic volume control voltage comprising a dioderectifier in circuit with said secondary, means for applying directcurrent generated by said rectifier to said amplifying means, andmeansin the circuit of said rectifier to cause said rectifier to beresponsive substantially only to recurring receivedv synchronizingimpulses, said diode rectifier having in connection therewith a threeelement thermionic combination adapted to draw current in the absence ofsignal and means for applying voltage therefrom to the cathode of saiddiode to bias it to out off in the absence of signal of predeterminedvalue.

10. In a facsimile installation for connection to a radio set, an inputtransformer having a primary and secondary, means for selectivelyconnecting said primary to said radio set and facsimile means inconnection with said secondary, a third winding on said transformer, andmeans in connection with said third winding for indicating a correctsignal level for facsimile reception, and switch means operating inconnection with said first mentioned switch means for placingsaid'indicating device in the circuit of said third winding when anadjustment is desired, there being amplifying means between saidfacsimile means and said secondary, and means for deriving an automaticvolume control voltage and for applying it to said amplifying means,located between said secondary and said amplifying means, said means forderiving an automatic volume control voltage comprising a dioderectifier in circuit with said secondary, means for applying directcurrent generated by said rectifier to said amplifying means, and meansin the circuit of said rectifier to cause said rectifier to beresponsive substantially only to recurring receivedsynchronizing-impulses, said diode rectifier having in connectiontherewith a three element thermionic combination adapted to draw currentin the absence of signal and means for applying voltage therefrom to thecathode of said diode to bias it to cut off in the absence of signal ofpredetermined value, and a connection between the control element, ofsaid thermionic combination and the circuit of said diode such as tobias said combination to cut off when said diode operates as arectifier.

FRANK ,L. WEDIG. JR.

